The present invention relates to a light-wave distance meter based on light pulses, and particularly to a light-wave distance meter based on light pulses which detects the time interval of return light pulses accurately by means of a relatively simple circuit arrangement and enables such a long distance measurement in which the noise dominates over the signal.
Conventional light-wave distance meters are designed to receive an echo light pulse returning from a corner cube prism that is placed at a target of measurement.
The basic principle of these conventional light-wave distance meters will first be explained with reference to FIG. 10. A light emission means is activated by an emission timing signal a', and it emits a light pulse b'. The radiated light pulse b' is reflected by the measurement target, and thereafter it is received by a light reception means and converted into a return pulse signal c'. The return pulse signal c' is converted analog to digital in response to clock pulses d', and the resulting data is stored in a memory. These operations are repeated for several emission light pulses b', and the stored data are averaged. The averaged data e' in the memory has its addresses for points on the waveform corresponding to measured values of the distance to the target, and the distance L from the distance meter to the target is calculated from the address A of the peak value of the data as follows. EQU L=(A*C)/(f.sub.s *2) (1)
where C is the velocity of light, and f.sub.s is the frequency of the clock signal d'. In the formulas herein, an asterisk (*) denotes multiplication.
However, the conventional light-wave distance meters based on light pulses have their measuring accuracy depending on the resolution of detection of the return light pulse, and therefore these distance meters need to have a sampling clock period for A/D conversion small enough relative to the width of the return pulse signal c' and need to have an A/D converter with a large number of bits. Specifically, in case the light emission means is a pulsed laser diode, which generally has a light pulse emission width of around 10 ns, the clock signal d' must have a frequency of several hundred MHz. The need of a very fast A/D converter results in an expensive distance meter.
Moreover, the conventional light-wave distance meters are designed to average stored data produced from several radiation light pulses b' after receiving the last radiation light pulse, and this calculation process imposes a great load to the computation means. Specifically, in the case of distance measurement based on 16 consecutive light pulses b' with a clock signal d' of 300 MHz for a maximum measurable distance of 1000 meters, the data averaging process needs to take place 2000 times, resulting in a long distance measuring time.
Although a conceivable technique for an enhancement of measuring accuracy is the interpolation process between two clock pulses d' for determining the centroid of the return pulse signal c', the distortion of the signal c' can be a problem. In case the return pulse signal c' includes frequency components above the range of detection by the clock signal d', a cyclic linearity error will emerge in the result of distance measurement. Specifically, in case the return pulse signal f' is a triangular wave as shown in FIG. 11, the execution of A/D conversion with clock pulses g' for the signal f' does not produce a difference in distance between data h' and i' in the memory, but it merely exhibits a variation in the quantity of light. It is generally very difficult to control the output waveform of a pulsed laser diode having a pulse width of several tens of ns, and therefore the linearity error of the distance measurement result is dependent on the characteristics of the laser diode as a light emission means.
On this account, there has been an earnest desire for a light-wave distance meter based on light pulses which does not use a fast expensive A/D converter, does not depend on the characteristics of a pulsed laser diode in regard to the linearity error, and reduces the time of distance measurement.